The invention relates to a supply system for a fuel cell stack with an adjustable humidifying device for humidifying a gas that is supplied to the fuel cell stack in response to a control signal, a dehumidifying device for dehumidifying and discharging exhaust gas formed from the supply gas after it has been conducted through the fuel cell stack, a controller for generating the control signal, and a warning device. The invention also provides a method for controlling such a supply system.
Fuel cell stacks provide a possible alternative for the mobile energy supply for motor vehicles in the near future. As is well known, in fuel cell stacks a fuel (often hydrogen) reacts electrochemically with an oxidant (often ambient air) and electric energy is generated or is converted from chemical energy. During the reaction, a membrane (often a proton exchange membrane, or “PEM”) which separates the fuel from the oxidant during the electrochemical process, plays a key role, for which purpose, it must be kept humid within defined limits during the operation of the fuel cell stack. Insufficient humidification can lead to drying out and premature wear of the membrane, and excessive humidification can cause a reduction of the performance of the fuel cell stack. The humidification is usually applied to input working gases.
German patent document DE 102 196 26 A1 discloses a fuel cell system, which uses a humidity sensor to control the humidification. The sensor measures humidity of the inflowing working gases, and actuates a control device (an adjustable bypass line with regard to the flow-through humidifier, upstream of the fuel cell stack) for controlling the humidity on the basis of the sensor signals. As an alternative to the humidity sensor, it is also suggested in this document to use a “virtual control” for controlling the humidity, wherein the control device is then actuated based on the performance of the fuel cell stack.
International patent document WO 02/23660 A2 suggests a different method or device for humidifying fuel cells or their membranes, in which a control device (a valve) is controlled based on the temperature of the cooling water fluid of the fuel cells.
Japanese patent document JP 2005149897 A (publication number), possibly the closest prior art, discloses a humidifying system for fuel cells which includes, similarly to the first-mentioned document, a control device in the form of a bypass line around a humidifying device. The control device is actuated by an actuator via a controller and the control signal is generated based on temperature measurements of the gas flowing towards the fuel cell system or of the gas exiting from the fuel cell system in a region in front of a dehumidifier.
One object of the invention is to provide a supply system and a warning device for a fuel cell stack and a method for controlling the supply system, which enables a reliable control of the humidity in the region of the membrane or the membranes of a fuel cell stack in a technically simple manner.
This and other objects and advantages are achieved by the supply system according to the invention, for a fuel cell stack which preferably has a plurality of fuel cells, especially in a PEM configuration (proton exchange membrane). For example, over 100 fuel cells are assembled to a stack.
The supply system has an adjustable humidifying device as an actuator, which is formed and/or configured to humidify a supply gas for the fuel cell stack, based on a control signal. The supply gas may be either cathode or anode gas, for example ambient air or fuel (hydrogen). The fuel may be supplied, without a reformer, from a tank which can be refueled.
The humidifying device increases the relative humidity of the supply gas. (Relative humidity is preferably defined as the percentage ratio between the momentary water vapor pressure and the saturated water vapor pressure.) The supply gas is especially completely saturated with water vapor with a relative humidity of 100%. The excess humidity often deposits as condensate or mist when 100% is exceeded. The increase of the relative humidity is adjusted based on the control signal, so that the actuator accessed by the control signal gives the amount of the relative humidity of the supply gas.
In addition, a dehumidifying device, which is integrated into the supply system, dehumidifies exhaust gas (used or partially used supply gas) from the fuel cell stack, so that a dehumidified exhaust gas is formed. Recyclable fluid, especially water, is withdrawn from the exhaust gas by the dehumidifying device, and can be returned to the supply system in a cycle, and thereby the fuel cell stack.
Finally, a controller is provided for generating the control signal which may be either electric (especially, digital), or mechanical.
According to the invention, the controller may be formed from a programming or switching point of view, to generate the control signal based on the gas temperature of the dehumidified exhaust gas. The controller preferably comprises a processor unit, as for example micro controller, DSP, ASIC or the like. For measuring the gas temperature of the dehumidified exhaust gas, a temperature sensor is preferably integrated in the supply system, directly at and/or near the outlet of the dehumidifying device, or is integrated in the humidifying device to avoid or minimize measuring errors.
In a practical implementation of the invention, the control signal is generated on the basis of the temperature difference between the exhaust gas and the dehumidified exhaust gas. For determining the temperature difference, a temperature difference sensor or two separate temperature sensors can for example be used. Instead of the temperature of the exhaust gas, a temperature can also be measured at another measuring point, as long as this is in a sufficiently predictable connection with the temperature of the exhaust gas. In other words, relative measurements for the temperature of the exhaust gas are also within the scope of the invention.
The invention has the advantage of providing a control (especially a regulation or control of the humidity at the fuel cell inlet or -outlet, and thus also at the membranes of the fuel cells) with an acceptable cost and space requirement.
The invention operates on the assumption that the relative humidity of the exhaust gas of the dehumidifying device decreases over the path of the dehumidifying device by the withdrawal of humidity from the exhaust gas, so that the exhaust gas is again enabled to take up liquid water by means of a renewed evaporation. However, this evaporation requires energy which is withdrawn from the exhaust gas, which leads to a cooling of the exhaust gas. The resulting temperature difference between the dehumidifying device inlet and the dehumidifying device outlet or between exhaust gas and dehumidified exhaust gas is proof that liquid water was present and was evaporated. The temperature difference is thereby higher, the more water is evaporated (that is, the more humid the exhaust gas from the fuel cell stack). When implementing the deliberation, it is possible to determine the outlet humidity of the exhaust gas from the fuel cell stack on the basis of the temperature of the dehumidified exhaust gas or on the basis of the temperature difference between exhaust gas and dehumidified exhaust gas sufficiently accurately. With knowledge of the fuel cell stack outlet humidity and further operating parameters, the inlet humidity of the supply gas at the fuel cell stack can then be estimated or determined. The temperature (or temperature difference) measurement thus forms a relative measurement for the inlet humidity of the supply gas at the fuel cell stack, so that it is possible to generate a control signal through the controller, which influences the humidifying device in such a manner that desired humidity conditions are adjusted for the fuel cell stack.
Further advantages compared to the known state of the art result in that tolerances caused by production of the fuel cell stack or the humidifying device can be considered by the controller or by a calibration of the controller, as the supply system provides a feedback regarding the actual operating state of the fuel cell stack.
In other words, the relative humidity of the exhaust gas coming from the fuel cell stack can be determined sufficiently accurately in a certain region via the connection of the gas humidity at the fuel cell stack outlet with the temperature difference between the fuel cell stack outlet and the dehumidifying device outlet. The supply system thereby also permits adjustment of the fuel cell stack inlet humidity with the help of the actuator of the adjustable humidifying device, in such a manner that the desired humidity level is adjusted at the fuel cell stack outlet and inlet.
In a preferred embodiment, a regulation or control of several magnitudes is implemented in the controller. Thus, the control signal is and/or can be generated using further input parameters, as for example fuel cell stack temperature, pressure, released fuel cell stack current and/or air stoichiometry, and/or further actuators as e.g. the cooling water temperature of the fuel cell stack, the system pressure and/or the air stoichiometry can be adjusted. With knowledge of the optional further inlet magnitudes it is for example possible to determine the connection between relative humidity at the fuel cell stack outlet and inlet more accurately.
In a preferred further embodiment of the invention, the humidifying device and the dehumidifying device are formed as a gas-to-gas humidifier, which especially has a passive functionality. An especially compact design of the supply system is achieved by this preferred further embodiment, and the fluid withdrawn from the exhaust gas is added sparingly to the supply gas as operating matter. The gas-to-gas humidifier has for example a humid region and a dry region, which are separated by a membrane, through which the liquid can diffuse. The dehumidifying device is especially formed in such a manner that liquid water is present in the humid region during operation, which can be absorbed again by the partially dehumidified gas.
The actuator of the humidifying device is formed as a bypass line with an adjustable valve device; preferably, the bypass line passes around the humidifying device. This embodiment considers that the humidifying or dehumidifying device or the gas-to-gas humidifier is optionally configured to the full load of the fuel cell stack, so that the air humidity of the fuel cell stack inlet is too high when partially loaded and is decreased with the help of the bypass line whose through-flow is controlled via the valve device. The valve device is thereby preferably formed as a throttling device.
In a preferred embodiment, the supply system is characterized by a working region for the relative humidity of the exhaust gas between 80% and 100%. On the one hand, this working region is preferable for a gentle operation of the membrane of the fuel cell stack, and on the other hand, the temperature measurements in this working region are especially significant.
In a robust implementation, a regulation circuit is provided for generating the control signal, wherein the variable is based on the gas temperature of the dehumidified exhaust gas and/or the temperature difference between exhaust gas and dehumidified exhaust gas or formed as this.
A next object of the invention is to provide a method for controlling the previously described supply system, wherein in a first operating mode a cooling water temperature for cooling the fuel cell stack in a load-dependent manner (that is, based on the generated current and/or the generated energy) is adjusted; and the control signal is controlled based on the temperature difference between the exhaust gas and dehumidified exhaust gas. In this first operating mode, the cooling water temperature is for example adjusted via definite grids or look-up tables based on the load, and the humidity at the fuel cell inlet is controlled or regulated with the actuator by means of the temperature difference. Thereby, the humidity in the fuel cell stack can be determined and controlled with the help of further operating conditions, as for example fuel cell stack current, air stoichiometry, cooling water temperature, pressure and the inlet temperature of the supply gas. The air stoichiometry characterizes the relation of the air amount supplied to the fuel cell to the air amount necessary for the combustion. The air mass flow can generally also be used as an operating parameter instead of the air stoichiometry. It is a special advantage of this first operating mode that the cooling water inlet temperature can be adjusted relatively quickly by means of a grid, and the humidity is only controlled by the actuator.
In an addition to the first operating mode, the system pressure (gas pressure) in the fuel stack is decreased as much as possible. The adjustment of the humidity takes place by means of the actuator, so that the highest possible system efficiency can be achieved with a desired humidity. Additional advantages of this modification can be seen in that the low system pressure also effects a low acoustic emission, and the supply system can be regulated dynamically, in a simple manner.
In a second possible operating mode, the cooling water temperature and the control signal are adjusted or regulated based on the temperature of the dehumidified exhaust gas and/or the temperature difference between exhaust gas and dehumidified exhaust gas. With this second operating mode, a desired humidity profile can be adjusted in the fuel cell stack, for example as in winter operation, with regard to the inlet and outlet humidity with a cooling water temperature as low as possible, which is positive for the durability of the individual fuel cells.
With further modifications of the second operating mode, the system pressure and/or the air stoichiometry are also adjusted, regulated and/or controlled dependent on the temperature of the dehumidified gas, and/or the temperature difference between exhaust gas and dehumidified gas. With this modification, advantages regarding a cooling water temperature as low as possible and thus prolonged fuel cell durability, a high system efficiency due to the low system pressure, and a low acoustic emission can be seen as advantages.
Summing up, the invention provides a system for the control, especially control or regulation of the humidity at the fuel cell stack inlet and/or outlet based on the temperature or the temperature difference between fuel cell stack outlet and dehumidifying device outlet. By connecting the air humidity at the fuel cell stack outlet with the temperature difference between the fuel cell stack outlet and dehumidifying device outlet, the humidity content of the exhaust gases coming from the fuel cell stack can be determined approximately in a certain working region. The fuel cell stack inlet humidity can be changed with the help of the actuator in such a manner that the desired humidity level adjusts itself at the fuel cell stack outlet and thereby inlet. The fuel cell stack outlet humidity is in direct connection with the inlet humidity, when the operating conditions such as temperature, pressure, fuel cell stack current and air stoichiometry are known. This control principle enables it to adjust different humidity levels in the fuel cell stack in the long run, as can for example be necessary to adjust for summer or winter operation (which differ regarding desired humidity levels at the outlet of the dry side of the humidifying device.) By sensing the temperature difference, a feedback is obtained in the vehicle during the operation of the fuel cell stack regarding the humidity state of the fuel cell stack and—if necessary—immediate measures or adjustments can be initiated. The operation of the fuel cell stack will thereby become more stable in the normal state, especially, fewer performance-reduced fuel cells occur, and a reaction can take place immediately in the case of a fault, which will be beneficial for the durability of the fuel cell. With an optional addition by a system pressure regulation, positive effects on the system efficiency can also be expected. The described measuring principle can thereby be realized in a relative cost-effective manner.
A next object of the invention which is based on the same inventive idea, is a warning device for a supply system for a fuel cell stack, with a dehumidifying device for dehumidifying exhaust gases exiting from the fuel cell stack to convert them to dehumidified exhaust gases, and with a or the controller for generating a control signal. According to the invention, the control signal is generated based on the temperature difference between the exhaust gas and the dehumidified exhaust gas. The warning device especially optionally has an arbitrary choice of the characteristics of the supply system described previously with regard to the measuring and evaluating principle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.